Hematopoietic stem cells (HSCs) in the bone marrow supply the circulation with mature blood cells throughout life. Division of the HSCs both replenishes their own numbers and provides committed progenitor cells that give rise to the erythrocyte, leukocyte, and megakaryocyte lineages. Progenitor cells undergo a limited number of divisions then differentiate into mature blood cells. Progenitor cell division and differentiation must be carefully balanced in order to provide the correct numbers and proportions of mature cells. Imbalances between proliferation and differentiation underlie common hematological problems like leukemia, lymphoma, and myelodysplastic syndromes. Despite their obvious importance, the mechanisms that control the choice between continued cell division and terminal differentiation are largely unknown. This project will examine the role of the regulatory protein Geminin in the proliferation and differentiation of hematopoietic cells. Geminin is required for pluripotency and is thought to maintain dividing cells in an undifferentiated state. Geminin is a bi-functional protein that controls the extent of DNA replication and inhibits homeodomain (Hox) transcription factors that are known to be crucial regulators of hematopoiesis. Geminin knockdown activates the Fanconi Anemia pathway, and over- expression of the replication factor inhibited by Geminin, Cdt1, in p53-/- T cells accelerates the development of lymphoblastic lymphoma. We are using a conditional knockout strategy to investigate the role of Geminin in definitive hematopoiesis. Gene-targeted mice, in which LoxP sites flank the essential exons of the Geminin gene, are being bred to mice that express Cre recombinase under the control of the interferon-inducible Mx-1 promoter. These mice will be treated with polyinosine- polycytidine (poly I:C) to induce Cre recombinase and delete the Geminin gene in hematopoietic cells. We will determine how Geminin loss affects the pattern of cell division and differentiation in the bone marrow, how Geminin regulates the cell cycle of hematopoietic cells, and whether Geminin suppresses the development of leukemias or lymphomas. We hope to gain a more complete understanding of normal hematopoiesis and identify a new target for therapy of hematological malignancies. PROJECT NARRATIVE Stem cells in the bone marrow continually divide throughout a person's life in order to produce mature blood cells. Problems in the division or the maturation of the growing blood cells give rise to leukemia and lymphoma, two of the most common cancers. This project examines the role of the protein Geminin in controlling blood cell development.